It is actually because your body can’t digest fiber that it plays such an important part in digestion. Soluble fiber, like that found in cucumbers, blueberries, beans, and nuts, dissolves into a gel-like texture, helping to slow down your digestion. This helps you to feel full longer and is one reason why fiber may help with weight control.
Insoluble fiber, found in foods like dark green leafy vegetables, green beans, celery, and carrots, does not dissolve at all and helps add bulk to your stool. This helps food to move through your digestive tract more quickly for healthy elimination. Many whole foods, especially fruits and vegetables, naturally contain both soluble and insoluble fiber.
Dr. Mercola is a great force for health, but fiber is nothing but a distraction. If folks want to eat it - great! Get some of this to really take care of that insufficient fiber problem: SNL Colon Blow
Dr. M cites the science that "shows" how good for you fiber is, but this is the same science that shows that low fat diets are good, and has been wrong 100% of the time when used for public health. Epidemiology can do amazing things, but is generally abused by an uninformed science press, writing to a mis-informed audience, and would better serve as fertilizer in many cases.
However, if you eat meat, vegetables, nuts and seeds, little fruit or starch, no sugar/no wheat, you are still doing what Dr. M recommends so it is a moot point.
If I could change anything, it would be that the discussion would not be fat or no fat, carbs or no carbs, fiber or no fiber - that is simplified and reductionist language. There are good and bad fats (saturated fats are great!), good and bad carbs (veggies, yumm), and good and bad fiber. However, even the good fiber is just part of the good foods we eat when we mimic a hunger gatherer diet.